The history of science is partly the history of an idea that is by now so familiar that it no longer astounds: the universe, including our own existence, can be explained by the interactions of little bits of matter. We scientists are in the business of discovering the laws that characterize this matter. We do so, to some extent at least, by a kind of reduction. The stuff of biology, for instance, can be reduced to chemistry and the stuff of chemistry can be reduced to physics.Thomas Nagel has never been at ease with this view. Nagel, University Professor of Philosophy and Law at New York University, is one of our most distinguished philosophers. He is perhaps best known for his 1974 paper, “What Is It Like to Be a Bat?,” a modern classic in the philosophy of mind. In that paper, Nagel argued that reductionist, materialist accounts of the mind leave some things unexplained. And one of those things is what it would actually feel like to be, say, a bat, a creature that navigates its environment via the odd (to us) sense of echolocation. To Nagel, then, reductionist attempts to ground everything in matter fail partly for a reason that couldn’t be any nearer to us: subjective experience. While not denying that our conscious experiences have everything to do with brains, neurons, and matter, Nagel finds it hard to see how these experiences can be fully reduced with the conceptual tools of physical science.In Mind and Cosmos, Nagel continues his attacks on reductionism. Though the book is brief its claims are big. Nagel insists that the mind-body problem “is not just a local problem” but “invades our understanding of the entire cosmos and its history.” If what he calls “materialist naturalism” or just “materialism” can’t explain consciousness, then it can’t fully account for life since consciousness is a feature of life. And if it can’t explain life, then it can’t fully account for the chemical and physical universe since life is a feature of that universe. Subjective experience is not, to Nagel, some detail that materialist science can hand-wave away. It’s a deal breaker. Nagel believes that any future science that grapples seriously with the mind-body problem will be one that is radically reconceived.As Nagel makes clear in the subtitle of Mind and Cosmos, part of what he thinks must be reconceived is our reigning theory of evolutionary biology, neo-Darwinism. Neo-Darwinism maintains, or at least implies, that the origin and history of life can be explained by materialist means. Once the first life arose on earth, the fate of the resulting evolutionary lineage was, neo-Darwinists argued, shaped by a combination of random mutation and natural selection. Biological types that survive or reproduce better than others will ultimately replace those others. While natural selection ensures that species constantly adapt to the changing environments around them, the process has no foresight: natural selection responds only to the present environment and evolution cannot, therefore, be aiming for any goal. This view, Nagel tells us, is “almost certainly false.”Before creationists grow too excited, it’s important to see what Nagel is not claiming. He is not claiming that life is six thousand years old, that it did not evolve, or that natural selection played no part in this evolution. He believes that life has a long evolutionary history and that natural selection had a part in it. And while he does believe that intelligent design creationists have asked some incisive questions, Nagel rejects their answers. Indeed he is an atheist. Instead Nagel’s view is that neo-Darwinism, and in fact the whole materialist view elaborated by science since the seventeenth century, is radically incomplete. The materialist laws of nature must, he says, be supplemented by something else if we are to fold ourselves and our minds fully into our science.His leading contender for this something else is teleology, a tendency of the universe to aim for certain goals as it unfolds through time. Nagel believes that (currently unknown) teleological laws of nature might mean that life and consciousness arise with greater probability than would result from the known laws of physics, chemistry, and biology.Scientists shouldn’t be shocked by Nagel’s claim that present science might not be up to cracking the mind-brain problem or that a profoundly different science might lie on the horizon. The history of science is filled with such surprising transformations. Nor should we dismiss Nagel’s claims merely because they originate from outside science, from a philosopher. Much the same thing happened when natural theology—the scientific attempt to discern God’s attributes from His biological handiwork—gave way to Darwinism.It was the philosopher David Hume who began to dismantle important aspects of natural theology. In a devastating set of arguments, Hume identified grievous problems with the argument from design (which claims, roughly, that a designer must exist because organisms show intricate design). Hume was not, however, able to offer an alternative account for the apparent design in organisms. Darwin worked in Hume’s wake and finally provided the required missing theory, natural selection. Nagel, consciously or not, now aspires to play the part of Hume in the demise of neo-Darwinism. He has, he believes, identified serious shortcomings in neo-Darwinism. And while he suspects that teleological laws of nature may exist, he recognizes that he hasn’t provided anything like a full theory. He awaits his Darwin.Mind and Cosmos is certainly provocative and it reflects the efforts of a fiercely independent mind. In important places, however, I believe that it is wrong. Because Nagel’s book sits at the intersection of philosophy and science it will surely attract the attention of both communities.1 As a biologist, I will perhaps inevitably focus on Nagel’s more scientific claims. But these are, it appears, the claims that are most responsible for the excitement over the book.I begin by considering the reasons Nagel believes that materialist science, including neo-Darwinism, is false. I then turn to his alternative theory, teleology.

2.

Nagel believes that materialism confronts two classes of problems. One, which is new to Nagel’s thought, concerns purported empirical problems with neo-Darwinism. The other, which is more familiar to philosophers, is the alleged failure of materialism to explain consciousness and allied mental phenomena.Nagel argues that there are purely “empirical reasons” to be skeptical about reductionism in biology and, in particular, about the plausibility of neo-Darwinism. Nagel’s claims here are so surprising that it’s best to quote them at length:

I would like to defend the untutored reaction of incredulity to the reductionist neo-Darwinian account of the origin and evolution of life. It is prima facie highly implausible that life as we know it is the result of a sequence of physical accidents together with the mechanism of natural selection. We are expected to abandon this naïve response, not in favor of a fully worked out physical/chemical explanation but in favor of an alternative that is really a schema for explanation, supported by some examples. What is lacking, to my knowledge, is a credible argument that the story has a nonnegligible probability of being true. There are two questions. First, given what is known about the chemical basis of biology and genetics, what is the likelihood that self-reproducing life forms should have come into existence spontaneously on the early earth, solely through the operation of the laws of physics and chemistry? The second question is about the sources of variation in the evolutionary process that was set in motion once life began: In the available geological time since the first life forms appeared on earth, what is the likelihood that, as a result of physical accident, a sequence of viable genetic mutations should have occurred that was sufficient to permit natural selection to produce the organisms that actually exist?

Nagel claims that both questions concern “highly specific events over a long historical period in the distant past, the available evidence is very indirect, and general assumptions have to play an important part.” He therefore concludes that “the available scientific evidence, in spite of the consensus of scientific opinion, does not in this matter rationally require us to subordinate the incredulity of common sense.”This conclusion is remarkable in a couple ways. For one thing, there’s not much of an argument here. Instead Nagel’s conclusion rests largely on the strength of his intuition. His intuition recoils from the claimed plausibility of neo-Darwinism and that, it seems, is that. (Richard Dawkins has called this sort of move the argument from personal incredulity.) But plenty of scientific truths are counterintuitive (does anyone find it intuitive that we’re hurtling around the sun at 67,000 miles per hour?) and a scientific education is, to a considerable extent, an exercise in taming the authority of one’s intuition. Nagel never explains why his intuition should count for so much here.As for his claim that evolutionary theory is somewhat schematic and that it concerns events that happened long ago, leaving indirect evidence, this is partly true of any historical science, including any alternative to neo-Darwinism, e.g., the one that Nagel himself suggests. In any case, a good part of the evidence for neo-Darwinism is not indirect but involves experiments in which evolutionary change is monitored in real time.2More important, Nagel’s conclusions about evolution are almost certainly wrong. The origin of life is admittedly a hard problem and we don’t know exactly how the first self-replicating system arose. But big progress has been made. The discovery of so-called ribozymes in the 1980s plausibly cracked the main principled problem at the heart of the origin of life. Research on life’s origin had always faced a chicken and egg dilemma: DNA, our hereditary material, can’t replicate without the assistance of proteins, but one can’t get the required proteins unless they’re encoded by DNA. So how could the whole system get off the ground?Answer: the first genetic material was probably RNA, not DNA. This might sound like a distinction without a difference but it isn’t. The point is that RNA molecules can both act as a hereditary material (as DNA does) and catalyze certain chemical reactions (as some proteins do), possibly including their own replication. (An RNAmolecule that can catalyze a reaction is called a ribozyme.) Consequently, many researchers into the origins of life now believe in an “RNA world,” in which early life on earth was RNA-based. “Physical accidents” were likely still required to produce the first RNA molecules, but we can now begin to see how these molecules might then self-replicate.Nagel’s astonishment that a “sequence of viable genetic mutations” has been available to evolution over billions of years is also unfounded.3 His concern appears to be that evolution requires an unbroken chain of viable genetic variants that connect the first living creature to, say, human beings. How could nature ensure that a viable mutation was always available to evolution? The answer is that it didn’t. That’s why species go extinct. Indeed that’s what extinction is. The world changes and a species can’t find a mutation fast enough to let it live. Extinction is the norm in evolution: the vast majority of all species have gone extinct. Nagel has, I think, been led astray by a big survivorship bias: the evolutionary lineage that led to us always found a viable mutation, ergo one must, it seems, always be available. Tyrannosaurus rex would presumably be less impressed by nature’s munificence.4

3.

While Nagel’s worries about neo-Darwinism are misplaced, he’s on somewhat firmer (or at least more familiar) ground when he turns to mental phenomena like consciousness. These are, after all, separate problems. A science might explain the evolution of life but leave consciousness—the subjective experience of the saltiness of popcorn, the shock of cold water, or the sting of pain—unaccounted for. Consciousness is Nagel’s big problem:

Consciousness is the most conspicuous obstacle to a comprehensive naturalism that relies only on the resources of physical science. The existence of consciousness seems to imply that the physical description of the universe, in spite of its richness and explanatory power, is only part of the truth, and that the natural order is far less austere than it would be if physics and chemistry accounted for everything.

Nagel’s story here starts, as it must, with Descartes. As Nagel writes, Descartes posited that matter and mind are “both fully real and irreducibly distinct, though they interact.” Given this, science was, from the outset, concerned solely with matter; mind belonged to a different domain. While scientists happily toiled under Cartesian dualism, giving rise to a recognizably modern science, philosophers often demurred. Instead, thinkers like Berkeley favored various forms of idealism, which maintains that nature is at bottom mind. Under idealism, then, any reductionist program would be in the business of collapsing matter to mind.Nagel argues that as a result of a rapid shift whose causes are unclear, these idealist philosophies were “largely displaced in later twentieth-century analytic philosophy by attempts at unification in the opposite direction, starting from the physical.” This approach likely seems natural to most of us. But we live with a tension. Though the materialist program of reducing mind to matter would appear the properly “scientific” approach, we haven’t the slightest idea how it would work. And it’s not for lack of trying. Philosophers have, Nagel reminds us, attempted many ways of tying mind to matter: conceptual behaviorism, physical identity theory, causal behaviorism, and functionalism, to name a few. To Nagel all these approaches have failed “for the same old reason”:

Even with the brain added to the picture, they clearly leave out something essential, without which there would be no mind. And what they leave out is just what was deliberately left out of the physical world by Descartes and Galileo in order to form the modern concept of the physical, namely, subjective appearances.

Nagel is deeply skeptical that any species of materialist reductionism can work. Instead, he concludes, progress on consciousness will require an intellectual revolution at least as radical as Einstein’s theory of relativity.Nagel’s chapter on consciousness is a concise and critical survey of a literature that is both vast and fascinating. He further extends his survey to other mental phenomena, including reason and value, that he also finds recalcitrant to materialism. (Nagel concludes that the existence of objective moral truths is incompatible with materialist evolutionary theory; because he is sure that moral truths exist, he again concludes that evolutionary theory is incomplete.)Nagel concedes that many philosophers do not share his skepticism about the plausibility of reducing mind to matter. And I can assure readers that most scientists don’t. I, however, share Nagel’s sense of mystery here. Brains and neurons obviously have everything to do with consciousness but how such mereobjects can give rise to the eerily different phenomenon of subjective experience seems utterly incomprehensible.Despite this, I can’t go so far as to conclude that mind poses some insurmountable barrier to materialism. There are two reasons. The first is, frankly, more a sociological observation than an actual argument. Science has, since the seventeenth century, proved remarkably adept at incorporating initially alien ideas (like electromagnetic fields) into its thinking. Yet most people, apparently including Nagel, find the resulting science sufficiently materialist. The unusual way in which physicists understand the weirdness of quantum mechanics might be especially instructive as a crude template for how the consciousness story could play out. Physicists describe quantum mechanics by writing equations. The fact that no one, including them, can quite intuit the meaning of these equations is often deemed beside the point. The solution is the equation. One can imagine a similar course for consciousness research: the solution is X, whether you can intuit X or not. Indeed the fact that you can’t intuit X might say more about you than it does about consciousness.And this brings me to the second reason. For there might be perfectly good reasons why you can’t imagine a solution to the problem of consciousness. As the philosopher Colin McGinn has emphasized, your very inability to imagine a solution might reflect your cognitive limitations as an evolved creature. The point is that we have no reason to believe that we, as organisms whose brains are evolved and finite, can fathom the answer to every question that we can ask. All other species have cognitive limitations, why not us? So even if matter does give rise to mind, we might not be able to understand how.To McGinn, then, the mysteriousness of consciousness may not be so much a challenge to neo-Darwinism as a result of it. Nagel obviously draws the opposite conclusion. But the availability of both conclusions gives pause.

4.

Given the problems that Nagel has with materialism, the obvious question is, What’s the alternative? In the most provocative part of Mind and Cosmos, he suggests one, teleology. While we often associate teleology with a God-like mind—events occur because an agent wills them as means to an end—Nagel finds theism unattractive. But he insists that materialism and theism do not exhaust the possibilities.Instead he proposes a special species of teleology that he calls natural teleology. Natural teleology doesn’t depend on any agent’s intentions; it’s just the way the world is. There are teleological laws of nature that we don’t yet know about and they bias the unfolding of the universe in certain desirable directions, including the formation of complex organisms and consciousness. The existence of teleological laws means that certain physical outcomes “have a significantly higher probability than is entailed by the laws of physics alone—simply because they are on the path toward a certain outcome.”Nagel intends natural teleology to be, among other things, a biological theory. It would explain not only the “appearance of physical organisms” but the “development of consciousness and ultimately of reason in those organisms.” Teleology would also provide an “account of the existence of the biological possibilities on which natural selection can operate.”Nagel concedes that his new theory isn’t fully fleshed out. He hopes merely to sketch the outlines of a plausible alternative to materialism. It’s unfortunate, though, that Mind and Cosmos is too brief to allow consideration of problems that attend natural teleology. For it seems to me that there are some, especially where the view confronts biology.Darwin himself wrestled with attempts to reconcile his theory with teleology and concluded, reluctantly, that it seemed implausible. While Darwin published almost nothing on such philosophical matters they loom large in his correspondence, particularly with Asa Gray, an American champion of evolution and a Christian. Gray, like Nagel, wanted to believe that, while Darwin had identified an important force in the history of life, nature also features teleology. In particular, Gray suggested that the variation provided by nature to natural selection biases the process in desirable directions.Darwin, though sometimes vacillating, argued that Gray’s reconciliation was implausible. Exercising his uncanny ability to discern deep truths in prosaic facts—in this case the artificial selection of a pigeon breed by a few fanciers—Darwin wrote Gray:

But I grieve to say that I cannot honestly go as far as you do about Design…. You lead me to infer that you believe “that variation has been led along certain beneficial lines”.—I cannot believe this; & I think you would have to believe, that the tail of the Fan-tail was led to vary in the number &direction of its feathers in order to gratify the caprice of a few men.5

Here’s another problem. Nagel’s teleological biology is heavily human-centric or at least animal-centric. Organisms, it seems, are in the business of secreting sentience, reason, and values. Real biology looks little like this and, from the outset, must face the staggering facts of organismal diversity. There are millions of species of fungi and bacteria and nearly 300,000 species of flowering plants. None of these groups is sentient and each is spectacularly successful. Indeed mindless species outnumber we sentient ones by any sensible measure (biomass, number of individuals, or number of species; there are only about 5,500 species of mammals). More fundamentally, each of these species is every bit as much the end product of evolution as we are. The point is that, if nature has goals, it certainly seems to have many and consciousness would appear to be fairly far down on the list.Similarly, Nagel’s teleological biology is run through with talk about the “higher forms of organization toward which nature tends” and progress toward “more complex systems.” Again, real biology looks little like this. The history of evolutionary lineages is replete with reversals, which often move from greater complexity to less. A lineage will evolve a complex feature (an eye, for example) that later gets dismantled, evolutionarily deconstructed after the species moves into a new environment (dark caves, say). Parasites often begin as “normal” complicated organisms and then lose evolutionarily many of their complex traits after taking up their new parasitic way of life. Such reversals are easily explained under Darwinism but less so under teleology. If nature is trying to get somewhere, why does it keep changing its mind about the destination?6I’ll be the first to admit that these problems may not be fatal. But they represent the sorts of awkward facts that occur immediately to any biologist. Minimally, they pose serious challenges to teleology, challenges that deserve, but do not receive, consideration in Mind and Cosmos.

5.

I will also be the first to admit that we cannot rule out the formal possibility of teleology in nature. It could turn out that teleological laws affect how the universe unfolds through time. While I suspect some might regard such heterodoxy as a crime against science, Nagel is right that there’s nothing intrinsically unscientific about teleology. If that’s the way nature is, that’s the way it is, and we scientists would need to get on with the business of characterizing these surprising laws. Teleological science is, in fact, more than imaginable. It’s actual, at least historically. Aristotelian science, with its concern for final cause, was thoroughly teleological. And the biological tradition that Darwinism displaced, natural theology, also featured a good deal of teleological thinking.The question, then, is not whether teleology is formally compatible with the practice of science. The question is whether the practice of science leads to taking teleology seriously. Nagel may find this question unfair. He is, he says, engaging in a “philosophical task,” not the “internal pursuit of science.” But it seems clear that he is doing more than this. He’s emphasizing purported “empirical reasons” for finding neo-Darwinism “almost certainly false” and he’s suggesting the existence of new scientific laws. These represent moves, however halting, into science proper. But science, finally, isn’t about defining the space of all formally possible explanations of nature. It’s about inference to the most likely hypothesis. And on these grounds there’s simply no comparison between neo-Darwinism (for which there is overwhelming evidence) and natural teleology (for which there is none). While one might complain that it’s unfair to stack up the empirical successes of neo-Darwinism with those of a new theory, this, again, gets the history wrong. Teleology is the traditional view; neo-Darwinism is the new kid on the block.None of this is to suggest that evolutionary biology will not, someday, change radically. Of course it might; any science might. Nor is it to suggest that materialism represents some final unassailable view and that teleology or, for that matter, theism will inevitably be spoken of in the past tense by many scientists. It is to say that the way to any such alternative view will have to acknowledge the full powers of present science. I cannot conclude that Mind and Cosmos does this.

NOTES:1. Nagel’s work has long attracted the attention of both philosophers and scientists. Indeed the careful reader will notice that I’m mentioned in his new book as a scientist-participant in a workshop that he organized on some of the topics covered in the book; many of the other participants were philosophers. ↩2. The field of “experimental evolution” is concerned with watching evolution as it occurs. Because of their short generation time, microbes are the focus of much of this work. ↩3. While I’ve heard this concern before, I must admit that I think I only now understand it. ↩4. This is not to say that adaptation is rare or that natural selection doesn’t modify the DNA sequences of species. Even species that ultimately go extinct have experienced many previous bouts of successful adaptation. ↩5. November 26, 1860; see www.darwinproject.ac.uk/entry-2998. Historians of science do not all agree that Darwin wholly banished teleology from his thinking; see the exchange between James G. Lennox (1993, 1994) and Michael T. Ghiselin (1994) in Biology and Philosophy. ↩6. It’s true that organisms are on average more complex now than they were three billion years ago. But as biologists have long recognized, this doesn’t require any inexorable bias toward complexity. If life starts from a floor of zero complexity, it can on average only get more complicated. ↩